U.S. patent application number 11/859300 was filed with the patent office on 2009-03-26 for method for fast and reliable fuel cell system start-ups.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Abdullah B. Alp, David A. Arthur, Balasubramanian Lakshmanan, Seth E. Lerner, John P. Salvador.
Application Number | 20090081491 11/859300 |
Document ID | / |
Family ID | 40471967 |
Filed Date | 2009-03-26 |
United States Patent
Application |
20090081491 |
Kind Code |
A1 |
Arthur; David A. ; et
al. |
March 26, 2009 |
Method for Fast and Reliable Fuel Cell System Start-Ups
Abstract
A method for providing a fast and reliable start-up of a fuel
cell system. The method uses a stack voltage response to a load to
assess if hydrogen and oxygen are being sufficiently distributed to
all of the fuel cells by coupling an auxiliary load to the fuel
cell stack until a predetermined minimum cell voltage has been
reached or a first predetermined time period has elapsed. The
method then determines whether a minimum cell voltage has dropped
to a first predetermined voltage and, if so, reduces the maximum
power allowed to be below the first predetermined voltage value,
determines whether the minimum cell voltage in the stack is below a
second predetermined voltage, or determines whether the minimum
cell voltage drop rate is greater than a predetermined voltage drop
rate. If none of these conditions are met, the method returns to
loading the stack with system components.
Inventors: |
Arthur; David A.; (Honeoye
Falls, NY) ; Salvador; John P.; (Penfield, NY)
; Lerner; Seth E.; (Honeoye Falls, NY) ;
Lakshmanan; Balasubramanian; (Pittsford, NY) ; Alp;
Abdullah B.; (West Henrietta, NY) |
Correspondence
Address: |
MILLER IP GROUP, PLC;GENERAL MOTORS CORPORATION
42690 WOODWARD AVENUE, SUITE 200
BLOOMFIELD HILLS
MI
48304
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
40471967 |
Appl. No.: |
11/859300 |
Filed: |
September 21, 2007 |
Current U.S.
Class: |
429/429 |
Current CPC
Class: |
H01M 8/04955 20130101;
H01M 8/0441 20130101; H01M 8/04402 20130101; H01M 8/04223 20130101;
H01M 8/04231 20130101; H01M 8/04447 20130101; H01M 2008/1095
20130101; H01M 8/04302 20160201; Y02B 90/10 20130101; Y02E 60/50
20130101; H01M 8/04753 20130101; H01M 8/04552 20130101; H01M
8/04619 20130101; H01M 8/04761 20130101 |
Class at
Publication: |
429/13 |
International
Class: |
H01M 8/02 20060101
H01M008/02 |
Claims
1. A method for starting a fuel cell system, said method
comprising: starting a compressor for providing cathode air to a
fuel cell stack; determining whether the fuel cell stack is filled
with air and requires an anode flush; starting an anode flow to the
anode side of the fuel cell stack; coupling an auxiliary load to
the fuel cell stack until a predetermined minimum cell voltage has
been reached by a fuel cell in the stack or a first predetermined
period of time has elapsed; coupling the fuel cell stack to system
loads; loading the stack with system components to a predetermined
maximum level for a second predetermined period of time; and
determining whether a minimum cell voltage in the stack is below a
first predetermined voltage value.
2. The method according to claim 1 further comprising reducing the
maximum power allowed from the stack if the minimum cell voltage is
below the first predetermined voltage value, determining whether
the minimum cell voltage in the stack is below a second
predetermined voltage value or the minimum cell voltage drop rate
is greater than a predetermined voltage drop rate.
3. The method according to claim 2 further comprising returning to
loading the stack with the system components if the minimum cell
voltage is not below the second predetermined voltage value or the
minimum cell voltage drop rate is not greater than the
predetermined voltage drop rate.
4. The method according to claim 2 further comprising determining
whether a maximum stack power allowed is less than a predetermined
power value if the minimum cell voltage is greater than the first
predetermined voltage value.
5. The method according to claim 4 further comprising determining
whether a battery can support another start-up sequence if the
maximum amount of power is less than the predetermined power value,
the minimum cell voltage is below the second predetermined voltage
value, or the minimum cell voltage drop rate is greater than the
predetermined voltage drop rate.
6. The method according to claim 5 further comprising limiting the
power drawn from the battery to a predetermined minimum value or a
maximum battery power available, whichever is smaller, if the
battery can support another start-up sequence.
7. The method according to claim 5 further comprising putting the
fuel cell system in a reduced performance mode if the battery
cannot support another start-up sequence.
8. The method according to claim 1 further comprising providing an
anode side flush using hydrogen gas.
9. The method according to claim 6 further comprising by-passing
the cathode air around the stack for a certain anode flow volume if
the stack has been flushed with hydrogen gas.
10. The method according to claim 1 wherein the fuel cell stack is
split fuel cell stacks that employ anode flow-shifting, and wherein
beginning an anode flow to the stack includes delivering the same
amount of gas flow to both of the split stacks simultaneously.
11. The method according to claim 2 wherein the first predetermined
voltage value is about 400 mV, the second predetermined voltage
value is about 200 mV and the predetermined cell voltage drop rate
is about 1000 mV/sec.
12. The method according to claim 1 wherein the predetermined
minimum cell voltage is about 700 mV and the first predetermined
period of time is about 10 seconds.
13. The method according to claim 1 wherein the auxiliary load is a
resistor.
14. A method for a start-up sequence for a fuel cell system
including first and second split stacks, said method comprising:
starting a compressor for providing cathode input air to the
cathode side of the split stacks; determining whether the split
stacks are filled with air and require an anode flush;
simultaneously starting an anode flow to the anode side of the
first and second split stacks; determining whether the start-up
sequence is in a first loop and the anode flush has been performed;
by-passing the split stacks with the cathode air if it is the first
time through the start-up sequence loop and the anode flush has
been performed for a predetermined number of anode flow volumes;
flowing the cathode air through the cathode side of the split
stacks if it is not the first start-up sequence loop or the anode
flush has not been performed; coupling an auxiliary load to the
split stacks until a predetermined minimum cell voltage has been
reached by a fuel cell in the split stacks or a first predetermined
period of time has elapsed; coupling the split stacks to system
loads; loading the split stacks with system components to a
predetermined maximum level for a second predetermined period of
time; determining whether a minimum cell voltage in the split
stacks is below a first predetermined voltage value; determining
whether a maximum stack power allowed is less than a predetermined
power value if the minimum cell voltage is greater than the first
predetermined voltage value; and updating a look-up table that
identifies how many anode volumes of hydrogen gas have been flowed
into the anode of the split stacks if the maximum stack power
allowed is greater than the predetermined power value.
15. The method according to claim 14 further comprising reducing
the maximum power allowed if the minimum cell voltage is below the
first predetermined voltage value, determining whether the minimum
cell voltage in the split stacks is below a second predetermined
voltage value or the minimum cell voltage drop rate is greater than
a predetermined voltage drop rate.
16. The method according to claim 14 further comprising returning
to loading the split stacks with the system components if the
minimum cell voltage is not below the second predetermined voltage
value or the minimum cell voltage drop rate is not greater than the
predetermined drop rate.
17. The method according to claim 14 further comprising determining
whether a battery can support another start-up sequence if the
maximum amount of power is less than the predetermined power value,
the minimum cell voltage is below the second predetermined voltage
value, or the minimum cell voltage drop rate is greater than the
predetermined voltage drop rate.
18. The method according to claim 17 further comprising limiting
the power drawn from the battery to a predetermined minimum value
or a maximum battery power available, whichever is smaller, if the
battery can support another start-up sequence.
19. The method according to claim 17 further comprising putting the
fuel cell system in a reduced performance mode if the battery
cannot support another start-up sequence.
20. The method according to claim 15 wherein the first
predetermined voltage value is about 400 mV, the second
predetermined voltage value is about 200 mV and the predetermined
cell voltage drop rate is about 1000 mV/sec.
21. The method according to claim 14 wherein the predetermined
minimum cell voltage is about 700 mV and the first predetermined
period of time is about 10 seconds.
22. The method according to claim 14 wherein the auxiliary load is
a resistor.
23. A method for starting a fuel cell system, said method
comprising: starting a compressor for providing cathode air to a
fuel cell stack; starting an anode flow to the anode side of the
fuel cell stack; and coupling an auxiliary load to the fuel cell
stack until a predetermined minimum cell voltage has been reached
by a fuel cell in the stack or a predetermined period of time has
elapsed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to a method for providing
fast and reliable fuel cell system start-ups and, more
particularly, to a method for providing fast and reliable fuel cell
system start-ups that includes coupling an auxiliary load to the
fuel cell stack at start-up until a predetermined minimum cell
voltage has been reached by a fuel cell in the stack or a
predetermined period of time has elapsed.
[0003] 2. Discussion of the Related Art
[0004] Hydrogen is a very attractive fuel because it is clean and
can be used to efficiently produce electricity in a fuel cell. A
hydrogen fuel cell is an electro-chemical device that includes an
anode and a cathode with an electrolyte therebetween. The anode
receives hydrogen gas and the cathode receives oxygen or air. The
hydrogen gas is dissociated in the anode to generate free protons
and electrons. The protons pass through the electrolyte to the
cathode. The protons react with the oxygen and the electrons in the
cathode to generate water. The electrons from the anode cannot pass
through the electrolyte, and thus are directed through a load to
perform work before being sent to the cathode.
[0005] Proton exchange membrane fuel cells (PEMFC) are a popular
fuel cell for vehicles. The PEMFC generally includes a solid
polymer-electrolyte proton-conducting membrane, such as a
perfluorosulfonic acid membrane. The anode and cathode typically
include finely divided catalytic particles, usually platinum (Pt),
supported on carbon particles and mixed with an ionomer. The
catalytic mixture is deposited on opposing sides of the membrane.
The combination of the anode catalytic mixture, the cathode
catalytic mixture and the membrane define a membrane electrode
assembly (MEA).
[0006] Several fuel cells are typically combined in a fuel cell
stack to generate the desired power. For the automotive fuel cell
stack mentioned above, the stack may include two hundred or more
fuel cells. The fuel cell stack receives a cathode reactant gas,
typically a flow of air forced through the stack by a compressor.
Not all of the oxygen is consumed by the stack and some of the air
is output as a cathode exhaust gas that may include water as a
stack by-product. The fuel cell stack also receives an anode
hydrogen reactant gas that flows into the anode side of the
stack.
[0007] A fuel cell stack typically includes a series of bipolar
plates positioned between the several MEAs in the stack, where the
bipolar plates and the MEAs are positioned between two end plates.
The bipolar plates include an anode side and a cathode side for
adjacent fuel cells in the stack. Anode gas flow channels are
provided on the anode side of the bipolar plates that allow the
anode reactant gas to flow to the respective MEA. Cathode gas flow
channels are provided on the cathode side of the bipolar plates
that allow the cathode reactant gas to flow to the respective MEA.
One end plate includes anode gas flow channels, and the other end
plate includes cathode gas flow channels. The bipolar plates and
end plates are made of a conductive material, such as stainless
steel or a conductive composite. The end plates conduct the
electricity generated by the fuel cells out of the stack. The
bipolar plates also include flow channels through which a cooling
fluid flows.
[0008] It has been proposed in the art to provide stack order
switching or anode flow-shifting in a fuel cell system that employs
split stacks. Particularly, valves and plumbing in the system are
provided so that the anode exhaust gas exiting a first sub-stack is
sent to the anode side of a second sub-stack, and the anode exhaust
gas exiting the second sub-stack is sent to the anode side of the
first sub-stack in a cyclical manner.
[0009] When a fuel cell system is shut down, un-reacted hydrogen
gas remains in the anode side of the fuel cell stack. This hydrogen
gas is able to diffuse through or cross over the membrane and react
with the oxygen in the cathode side. As the hydrogen gas diffuses
to the cathode side, the total pressure on the anode side of the
stack is reduced below ambient pressure. This pressure differential
draws air from ambient into the anode side of the stack. When the
air enters the anode side of the stack it generates a hydrogen/air
front that creates a short circuit in the anode side, resulting in
a lateral flow of hydrogen ions from the hydrogen flooded portion
of the anode side to the air-flooded portion of the anode side.
This high ion current combined with the high lateral ionic
resistance of the membrane produces a significant lateral potential
drop (.about.0.5 V) across the membrane. This produces a local high
potential between the cathode side opposite the air-filled portion
of the anode side and adjacent to the electrolyte that drives rapid
carbon corrosion, and causes the carbon layer to get thinner. This
decreases the support for the catalyst particles, which decreases
the performance of the fuel cell.
[0010] At the next system start-up, assuming enough time has gone
by, both the cathode and anode flow channels are generally filled
with air. When hydrogen is introduced into the anode flow channels
at system start-up, the hydrogen pushes out the air in the anode
flow channels also creating a hydrogen/air front that travels
through the anode flow channels. The hydrogen/air front causes a
catalytic reaction along the length of the membrane in each fuel
cell as the front moves that, in combination with the reaction
across the membrane, creates a high electric voltage potential.
This combined electric voltage potential is high enough to severely
degrade the catalyst and the carbon particles on which the catalyst
is formed, reducing the life of the MEAs in the fuel cell stack.
Particularly, the reaction created by the hydrogen/air front in
combination with the normal fuel cell reaction is orders of
magnitude greater than only the fuel cell reaction across the
membrane. For example, it has been shown that without addressing
the degradation effects of the hydrogen-air front at system
start-up, it only takes about 100 shutdown and start-up cycles to
destroy the fuel cell stack in this manner.
[0011] It has been proposed in the art to reduce the degradation
effect of the hydrogen/air front at system start-up by forcing
hydrogen through the anode flow channels as quickly as possible so
as to reduce the time that the degradation occurs. It has also been
suggested to introduce hydrogen into the anode flow channels at a
slow rate to provide active mixing of the air and hydrogen to
eliminate the hydrogen/air front. It has also been proposed in the
art to cool the fuel cell before removing the hydrogen from the
anode flow channels. However, all of these solutions have not
reduced the hydrogen/air degradation enough to provide a desired
lifetime of the fuel cell stack. Particularly, moving the
hydrogen/air front quickly does not completely eliminate the
degradation of the catalyst, and requires over-sized piping and
other components to rapidly purge the air from the anode flow
channels. Introducing the hydrogen slowly at start-up has the
disadvantage of requiring a recirculation pump that takes several
minutes to completely remove the air from the anode flow channels.
Further, requiring precise control of the amount of hydrogen into
the anode flow channels is difficult to implement.
[0012] It has also been proposed in the art to provide a load
across the fuel cell stack, such as a resistor, to reduce the
electric potential generated by the hydrogen/air front. However, an
extremely low resistance load will require electrical components
with a high power rating. Also, flow and balancing between cells in
a fuel cell stack can result in corrosion to the cell anodes.
Furthermore, in most embodiments, a resistor is typically not
sufficient on its own to minimize carbon corrosion.
[0013] The ideal fuel cell system start-up method purely from a
speed and reliability perspective would be to flow hydrogen at a
very high flow rate through the split stacks in parallel and then
out of the anode exhaust. Reliability is a function of cell
voltage, and those cells at system start-up that included
significant air in the anode flow channels could cause a very low,
possibly negative, cell voltage. The flow rate would be high enough
that any water blocking the anode flow fields will be forced out of
the stack. Also, any start-up degradation from the hydrogen/air
front would be low because the front speed would be so fast.
However, a problem exists in that the anode exhaust may have a
relatively high concentration of hydrogen, possibly causing a
combustible mixture. Such high anode flow rates would therefore
require additional system components, such as combustors,
accumulators, etc., resulting in a complex system.
[0014] It has been proposed in the art to fill the anode manifold
with hydrogen at system start-up, and then evenly flow hydrogen gas
through the anode flow channels. However, it has been shown that
this type of purge does not alone provide even hydrogen flow
through the cells, so additional actions may be needed. This is a
result of air still existing in the hydrogen flow channels,
especially at cold start.
SUMMARY OF THE INVENTION
[0015] In accordance with the teachings of the present invention, a
method for providing a fast and reliable start-up of a fuel cell
system is disclosed. The method includes starting a compressor that
provides cathode air to the cathode side of a fuel cell stack to
provide dilution air for the hydrogen exhaust. The method then
determines if the stack is filled with air and, if so, performs a
stack flush with hydrogen gas. The method then begins a hydrogen
flow to the anode side of the stack and a cathode airflow to the
cathode side of the stack. The method then uses a stack voltage
response to a load to assess if hydrogen and oxygen are being
sufficiently distributed to all of the fuel cells by coupling an
auxiliary load to the fuel cell stack until a predetermined minimum
cell voltage has been reached or a first predetermined time period
has elapsed. The method then couples the stack to system components
to load the stack to a predetermined maximum level for a
predetermined period of time. The method then determines whether a
minimum cell voltage in the stack has dropped to a first
predetermined voltage and, if not, determines whether the stack has
reached a maximum allowed power. If the minimum cell voltage has
reached the first predetermined voltage value, then the method
reduces the maximum power allowed to be below the first
predetermined voltage value, determines whether the minimum cell
voltage in the stack is below a second predetermined voltage, or
determines whether the minimum cell voltage drop rate is greater
than a predetermined voltage drop rate. If none of these conditions
are met, the method returns to loading the stack with system
components.
[0016] Additional features of the present invention will become
apparent from the following description and appended claims taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic block diagram of a fuel cell system
employing anode flow-shifting; and
[0018] FIGS. 2A and 2B are a flow chart diagram showing a process
for providing a fast and reliable fuel cell system start-up,
according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0019] The following discussion of the embodiments of the invention
directed to a method for reliably and quickly starting a fuel cell
system is merely exemplary in nature, and is in no way intended to
limit the invention or its applications or uses.
[0020] FIG. 1 is a schematic block diagram of a fuel cell system 10
including a first split fuel cell stack 12 and a second split fuel
cell stack 14. A compressor 16 provides cathode input air on
cathode input line 18 to the split stacks 12 and 14 through a
normally closed cathode input valve 20. Cathode exhaust gas is
output from the split stack 12 on line 24 and cathode exhaust gas
is output from the split stack 14 on line 26 where the cathode
exhaust gas is combined into a single cathode output line 28. A
normally closed cathode back pressure valve 30 controls the flow of
the cathode exhaust gas through the line 28. A cathode by-pass line
32 between the input line 18 and the output line 28 allows the
cathode input air to by-pass the stacks 12 and 14. A normally
closed by-pass valve 34 controls whether the cathode air by-passes
the stacks 12 and 14. If the valves 20 and 30 are closed and the
valve 34 is open, then air from the compressor 16 will by-pass the
stacks 12 and 14. Typically, a cathode humidification unit (not
shown) will be provided at a suitable location in the cathode input
line 18.
[0021] In this non-limiting embodiment, the split stacks 12 and 14
employ anode flow-shifting where the anode reactant gas flows back
and forth through the split stacks 12 and 14 at a predetermined
cycle in a manner that is well understood to those skilled in the
art. An injector 38 injects hydrogen gas from a hydrogen gas source
40 through anode line 42 to the split stack 12 and an injector 44
injects hydrogen gas from the hydrogen source 40 through anode line
48 to the split stack 14 in an alternating sequence. A connector
line 54 connects the anode sides of the split stacks 12 and 14.
[0022] A water separator 60 is coupled to the connector line 54 and
collects water in the anode gas flow between the split stacks 12
and 14. A normally closed drain valve 62 can be employed that is
periodically opened to vent the water to the cathode exhaust gas
line 28 on line 64. Further, an anode exhaust gas purge valve 66
can be provided in the connection line 54.
[0023] As discussed above, it is desirable to periodically bleed
the anode side of the split stacks 12 and 14 to remove nitrogen
that may otherwise dilute the hydrogen and affect cell performance.
Normally closed bleed valves 50 and 52 are provided for this
purpose. When an anode bleed is commanded, the bleed valve 50 or 52
is opened and the bled anode exhaust gas is sent to the cathode
exhaust gas line 28 depending on which direction the hydrogen gas
is currently flowing. Particularly, if the hydrogen gas is being
injected into the split stack 12 from the source 40 when a bleed is
triggered, then the bleed valve 52 is opened. Likewise, if the
hydrogen gas is being injected into the split stack 14 from the
source 40 when a bleed is triggered, then the bleed valve 50 is
opened. The flow-shifting will typically occur several times during
a normal bleed duration so that the bleed valves 50 and 52 have to
be opened and closed several times in time with the flow
switching.
[0024] The fuel cell stacks 12 and 14 generate current. During
normal stack operation, the current generated by the stacks 12 and
14 is used to drive system loads, such as an electrical traction
system (ETS) 56 on a vehicle. During a shut-down sequence, the
current generated by the stacks 12 and 14 may be used to charge a
battery 58, or be dissipated by other system components, and then
be dissipated by a resistor 68.
[0025] At one system shut-down sequence, the compressor 16 is
stopped and the valves 20 and 30 are closed to seal the cathode
side of the stacks 12 and 14. The flow of hydrogen is continued so
that any remaining oxygen in the stacks 12 and 14 is consumed. When
the stack power drops to a predetermined level, the current
generated by the split 12 stacks and 14 is switched from the ETS 56
to the battery 58. When the stack power decreases to another
predetermined level, the stack load is switched to the resistor 68.
Particularly, once the voltage has degraded to a fixed cut-off
voltage, the stack load is switched to the resistor 68. The cut-off
voltage could be the lower limit of a DC/DC converter (not shown),
or the lower limit of a power device. The objective of the battery
load is to consume and/or store any energy that otherwise would
have been wasted. It also reduces the energy consumption
requirements of the resistor load.
[0026] Once the oxygen has been consumed from the split stacks 12
and 14, the hydrogen flow is turned off and the valves 50, 52, 62
and 66 are closed to seal the anode side of the stacks 12 and 14.
When the system 10 is shut-down in this manner, the stacks 12 and
14 include an N.sub.2/H.sub.2 mixture in both the cathode side and
the anode side. Over time, air will leak into the stacks 12 and 14,
and the hydrogen in the stack 12 and 14 will initially consume the
oxygen. Additionally, the hydrogen will slowly leak out of the
stacks 12 and 14. As a result, the composition of the gases within
the stacks 12 and 14 will vary over time between a hydrogen rich
mixture in nitrogen and water to an air mixture.
[0027] The amount of hydrogen that is used to purge the split
stacks 12 and 14 can be calculated based on the volume of the anode
side of the stacks 12 and 14, the temperature of the stacks 12 and
14 and the pressure within the split stacks 12 and 14. The hydrogen
flow into the stacks 12 and 14 should be roughly one anode volume.
If an insufficient amount of hydrogen flows into the stack, some of
the fuel cells might be left containing an H.sub.2/O.sub.2 front.
If too much hydrogen flows into the first stack, excess hydrogen is
wasted to the exhaust and might enter into the second stack through
compression leading to a stagnant hydrogen/air front causing
excessive voltage degradation. The loop volume for each of the
stacks 12 and 14 is calculated and this information is combined
with the hydrogen flow rate during the start-up to determine the
purge time for the first stack.
[0028] FIGS. 2A and 2B are a flow chart diagram 70 showing a method
for quickly and reliably starting the fuel cell system 101
especially during cold starts, according to an embodiment of the
present invention. At box 72, the compressor 16 is started for
hydrogen output dilution purposes. The initial part of the system
start-up includes starting the compressor 16 to provide dilution
air for hydrogen that collects in the exhaust as a result of the
start-up sequence. The algorithm then determines whether the split
stacks 12 and 14 are filled with air at decision diamond 74 as a
result of the time they have been shut-down, and if so, initiates a
stack flush using a header purge at box 76. This provides a
technique for removing air and nitrogen from the header of both of
the stacks 12 and 14 prior to the stack flush. After the header has
been purged, the stack flush provides a large flow rate of hydrogen
gas through the anode flow fields to minimize start-up degradation
due to the hydrogen/air front, as discussed above.
[0029] The algorithm then begins the anode flow by opening the
header valve to the split stacks 12 and 14 in a 50/50 manner to
fill the anode flow channels with hydrogen gas at box 78. In this
flow process, both of the injectors 38 and 44 are used at the same
time to flow hydrogen gas evenly through the split stacks 12 and
14. All large valves are closed at this stage to allow for a well
controlled, low flow rate hydrogen injection. The valves that are
open typically have a small orifice, or large valves can be used
that are pulse width modulated to effectively provide a small
valve. The hydrogen injectors 38 and 44 are typically controlled
based on the anode outlet pressure of the split stacks 12 and 14.
However, in this case, the injectors 38 and 44 will switch modes to
flow control where the flow will be metered so that it is as high
as possible without causing exhaust emissions to exceed a
predetermined hydrogen concentration when mixed with the cathode
exhaust. Therefore, the hydrogen flow rate would be varied in real
time based on cathode dilution flow.
[0030] If the stack is not filled with air at the decision diamond
74, then the algorithm skips the stack flush step at the box 76,
and proceeds directly to the step of providing the anode flow at
the box 78.
[0031] At the same time, there should be a peak anode pressure to
cap the injectors 38 and 44. In other words, the cathode exhaust
flow rate needs to be known and the anode flow rate will be
estimated based on the injector duty cycle. The injectors 38 and 44
should be controlled so as to trigger as high a flow as possible
for emissions less than the predetermined threshold, and so that
anode pressures do not exceed a predetermined pressure, such as 150
kPa. The duration of this flow is determined based on a look-up
table in the software that takes the time since the last shut-down
as the input, and outputs a particularly minimum number of anode
volumes of hydrogen gas that should be flowed.
[0032] The algorithm then determines whether this is the first time
through the start loop at decision diamond 80, and whether the
anode side flush was skipped at the box 76, meaning that the most
recent shut-down time was not too far back in the past where the
anode flow channels are still significantly filled with hydrogen
gas. If both of these conditions are met, then the algorithm
by-passes the cathode air around the stacks 12 and 14 for some
duration of the anode flow, such as half, at box 82. When
by-passing the cathode air around the split stacks 12 and 14,
additional air is not added to the cathode side that may permeate
through the membranes to the anode side increasing the potential
for the damaging hydrogen/air front. In other words, it is
desirable to introduce hydrogen gas into the anode side before air
is introduced into the cathode side so that hydrogen permeates
through the membrane instead of air, reducing the hydrogen/air
front on the anode side of the stacks 12 and 14.
[0033] Once the cathode air has by-passed the stacks 12 and 14 for
the predetermined anode volume flow, the algorithm then flows the
cathode air through the stacks 12 and 14 for the remainder of the
anode flow at box 84. If this is not the first time through the
control loop or the stack flush did not occur at the box 76, then
the algorithm proceeds directly to flowing the cathode air through
the stacks 12 and 14 at box 86.
[0034] Next, the algorithm continues with the anode flow and
engages the pull-down resistor 68 coupled to the split stacks 12
and 14 as a load at box 88 until one of two conditions is met,
namely, that the minimum cell voltage is greater than a
predetermined voltage value, such as 700 mV, or a predetermined
period of time has elapsed, such as 10 seconds. By putting a load
on the split stacks 12 and 14, a voltage drop occurs across the
stacks 12 and 14 that more nearly matches the high voltage bus line
(not shown) coupled to the high voltage battery 58 in the system
10. Particularly, the algorithm uses a stack voltage response to a
load to assess if hydrogen and oxygen are being sufficiently
distributed to all of the fuel cells by coupling an auxiliary load
to the fuel cell stack. This step is one of the ways that the
algorithm provides a fast and reliable start-up by making sure that
the minimum cell voltage is high enough or enough hydrogen is in
the anode flow channels so that the operation of the stacks 12 and
14 is stable. If the stacks 12 and 14 are healthy, and no problems
exist, then the algorithm will proceed very quickly through these
steps of the control loop. However, if the split stacks 12 and 14
have significantly aged, or degraded for some other reason, then
the time period that the algorithm waits during the start-up
sequence will provide a better situation for the stacks 12 and 14
to start in a stable manner.
[0035] Once the minimum cell voltage is greater than the
predetermined voltage value or the predetermined time period has
expired, the algorithm then closes the stack contactors to the high
voltage bus line at box 90 to allow the split stacks 12 and 14 to
operate under the normal loads of the system 10. The algorithm then
loads the split stacks 12 and 14 at box 92 with as many of the fuel
cell system components as it can up to the maximum limit of the
split stacks 12 and 14 for a predetermined period of time, such as
seven seconds, to test the split stacks 12 and 14 and see if they
will operate normally.
[0036] The algorithm then determines whether the minimum cell
voltage has dropped to a predetermined voltage, such as 400 mV, at
decision diamond 94. If the minimum cell voltage in either of the
split stacks 12 or 14 is below the predetermined voltage, then the
reliability of the start-up is reduced. The algorithm then proceeds
to minimize the maximum power allowed to be drawn from the split
stacks 12 and 14 at box 96 in an attempt to try and raise the
minimum cell voltage above the predetermined value.
[0037] The algorithm also determines whether the minimum cell
voltage has dropped below another lower predetermined voltage, such
as 200 mV, or the minimum cell voltage drop rate is exceeding a
predetermined voltage drop rate, such as 1000 mV/sec, at decision
diamond 98. If neither of these two conditions is met, then the
algorithm returns to the box 92 to give the split stacks 12 and 14
another attempt to raise their minimum cell voltage above the first
predetermined voltage value.
[0038] If the minimum cell voltage is not less than the first
predetermined voltage value at the decision diamond 94, then the
split stack 12 or 14 may be operating properly. The algorithm then
determines whether the maximum power allowed from the split stacks
12 and 14 is less than a predetermined value, such as 90 kW, at
decision diamond 100. If the maximum stack power is below the
predetermined value, then the split stacks 12 and 14 have not
raised their maximum power output quick enough during the start-up
sequence, meaning that the split stacks 12 and/or 14 may be
unstable.
[0039] If the minimum cell voltage is less than the second
predetermined voltage value or the minimum cell voltage drop rate
is greater than the predetermined voltage drop rate at the decision
diamond 98, or the split stacks 12 and 14 have not reached the
maximum power allowed at the decision diamond 100, then the
algorithm determines whether the battery 58 can support another
loop through the start-up sequence at decision diamond 102. If
there is sufficient battery power and the number of iterations
through the loop has been less than a predetermined value, such as
eight, then the stack contactors are opened at box 104. Further,
the algorithm limits the maximum power draw from the battery 58 to
some predetermined maximum value, such as 20 kW, or to the maximum
battery power available, whichever is smaller, at box 106. The
algorithm then proceeds to the step of providing the anode flow to
the split stacks 12 and 14 at the box 78, where the answer to
whether this is the first time through the loop at the decision
diamond 80 will be no, increasing the number of performed
iterations through the loop.
[0040] If the battery 58 cannot support another iteration through
the loop or the maximum number of iterations through the loop has
been reached at the decision diamond 102, then the system 10 is put
in a reduced performance mode at box 108 that allows the vehicle to
operate, but with limited power, so that it can be driven to a
service station or other safe location.
[0041] If the maximum power allowed is greater than the
predetermined value at the decision diamond 100, then the algorithm
modifies the look-up table that identifies how many anode volumes
of hydrogen have been flowed into the anode flow field at box 110.
If the amount of anode flow needed is higher, then the table is
updated permanently in the software for the system. In this way,
the start time may be extended in the future for the new times
since the last shut-down, but the reliability of the system is
improved. Essentially, the table will adapt as the stack ages. Once
the table is updated, the algorithm will go to full system
operation and begin anode flow-shifting at box 112.
[0042] In alternate embodiments, the steps at the box 96 and the
decision diamonds 98 and/or 100 can be eliminated, and the no
decision from the decision diamond 94 can go to the decision
diamond 102. Further, the steps of the decision diamond 102 and the
boxes 104 and 106 can also be removed where the no decision of the
decision diamond 94 returns to the box 92. Also, the steps below
the close contactors at box 90 can be eliminated for a reduced
length start-up process.
[0043] The foregoing discussion discloses and describes merely
exemplary embodiments of the present invention. One skilled in the
art will readily recognize from such discussion and from the
accompanying drawings and claims that various changes,
modifications and variations can be made therein without departing
from the spirit and scope of the invention as defined in the
following claims.
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